I have an S250 and empty it is still heavy. I would guess that foam absorbs moisture over time and that the accessories that are installed are not included in the base weight. While the access panels may seem light weight I have handled them off the shelter and they are not as light as you would think.
I have an S280 on my deuce and it is listed as weighing 1700 empty and I can tell you that is BS. Perhaps a brand new one, with no water accumulated inside the skin and absolutely without attachments or accessories, and this is for a day or two it will weigh that amount.
In this discussion, we are concentrating the weight, and the sizes of things. I would suggest concentrating on what the goals are and then figuring out what military equipment could fill the bill. Granted most of the objectives were spelled out, but I have a few questions.
How much ground clearance do you need?
How much reall offroading do you foresee doing with this rig in really difficult conditions.
By answering those additional questions you can better determine if a 4000# trailer is towable behind your rig.
Another option would be to consider finding a shelter for the back of a deuce or some other truck that is simply a shell, not insulated as much as the S250 and 280 and in that way save the extra weight that the internal bracing and such create and also a more flexible shelter to adapt to a platform such as a Military trailer. As I recall I have seen them in the classifieds from time to time. Just an idea.
RL